The present invention relates to a process for alkylating aromatics.
The present invention is directed to a process for aromatics alkylation and in particular the alkylation of benzene with ethylene to provide ethylbenzene. Existing processes for producing ethylbenzene inherently produce polyalkylated species as well as the desired monoalkyated product. It is therefore normal to transalkylate the polyalkylated species with benzene to produce additional ethylbenzene either by recycling the polyalkylated species to the alkylation reactor or by feeding the polyalkylated species to a separate transalkylation reactor. It is, however, found that para-diethylbenzene (p-DEB) is easier to transalkylate than the ortho and meta isomers. There is therefore a continuing need for an aromatics alkylation process in which the polyalkylated by-products are rich in the para dialkyl component. Such a process would be particularly useful where it employs a catalyst which exhibits both alkylation and transalkylation activity under similar operating conditions inasmuch as the alkylation and transalkylation steps could be carried out in a single reactor with recycle of diethylbenzene.
U.S. Pat. No. 3,751,504 to Keown et al. and U.S. Pat. No. 3,751,506 to Burress describe vapor phase alkylation of aromatic hydrocarbons with olefins, e.g., benzene with ethylene, in the presence of ZSM-5 zeolite.
U.S. Pat. No. 4,086,287 to Kaeding et al. discloses a method for selectively ethylating monoalkyl benzenes to para-dialkyl substituted benzenes, such as para-diethylbenzene using ethylene as alkylating agent, over a zeolite-containing catalyst, e.g., ZSM-5, which may be incorporated in a matrix such as clay, silica, and/or metal oxides. The zeolite may be modified by incorporating a difficultly reducible oxide such as oxides of phosphorus, boron, and/or magnesium.
U.S. Pat. No. 4,117,026 to Haag et al. discloses a method for selectively producing para-dialkyl substituted benzenes from monoalkyl substituted benzenes, e.g., toluene and C2-C15 olefin, over a zeolite-containing catalyst, e.g., ZSM-5, which may be incorporated in a matrix such as clay, silica, and/or metal oxides. The reference further teaches enhanced para-xylene selectivity with large crystal ZSM-5 (1 micron) compared to small crystal ZSM-5 (0.03 micron).
U.S. Pat. No. 4,169,111 to Wight teaches a method for producing ethylbenzene by alkylating benzene with ethylene in the presence of a catalyst, e.g., ZSM-5 which can be bound with mineral oxide binder, e.g., alumina gel or silica gel. The process produces ethylbenzene and polyethylbenzenes. At least a portion of diethylbenzene fraction is recycled to the alkylation zone, while the remainder thereof plus the higher polyethylbenzenes are transalkylated with benzene to produce additional ethylbenzene.
U.S. Pat. No. 5,243,117 to Chang et al. discloses a method for selectively producing para-dialkyl substituted benzenes using a silica-modified non-organic ZSM-5 catalyst treated with an organosilicon compound, e.g., phenylmethylsilicone. The catalyst may be silica-bound.
U.S. Pat. No. 5,530,170 to Beck et al. discloses a method for alkylating ethylbenzene with ethylene to selectively produce para-diethylbenzene using a ZSM-5 zeolite catalyst which has been selectivated by multiple treatments with a siliceous material, e.g., dimethylsilicone. The catalyst may be silica-bound.
U.S. Pat. No. 5,689,027 to Abichandani et al. discloses a method for converting ethylbenzene to compounds that may be removed from an aromatic hydrocarbon stream with minimal xylene loss using a silica bound intermediate pore size zeolite that has been selectivated.
According to the invention it has now been found that a silica bound, small crystal ZSM-5 zeolite catalyst provides a higher p-DEB content in total DEB when used in the ethylation of benzene, thereby facilitating subsequent DEB conversion to ethylbenzene (EB). The novel catalyst also allows reduced alkylation temperature and lower xylene make as compared to conventional catalysts.
In one aspect, the present invention relates to a process for alkylating aromatics which comprises contacting an aromatic-containing feed with alkylating agent in the presence of a silica-bound ZSM-5 zeolite catalyst, wherein the ZSM-5 has a crystal size no greater than 0.05 micron, e.g., 0.02 to 0.05 micron, said contacting being conducted under alkylating conditions to provide a product containing a monoalkylated aromatic fraction and a polyalkylated aromatic fraction, wherein the polyalkylated aromatic fraction contains at least 40 wt. % of the para-dialkylaromatic species.
In another aspect, the present invention relates to a process for preparing a monoalkylated aromatic compound which comprises:
i) contacting an aromatic-containing feed in an alkylating reactor with alkylating agent in the presence of a silica-bound ZSM-5 zeolite alkylation catalyst, wherein the ZSM-5 has a crystal size of no greater than 0.05 micron, e.g., 0.02 to 0.05 micron, said contacting being conducted under alkylating conditions to provide a product containing a monoalkylated aromatic fraction and a polyalkylated aromatic fraction, wherein the polyalkylated aromatic fraction contains at least 40 wt % of the paradialkylaromatic species;
ii) separating the polyalkylated aromatic fraction from the product, and
iii) contacting the polyalkylated aromatic fraction in the presence of unsubstituted aromatic and a transalkylation catalyst under transalkylating conditions to provide a monoalkylated aromatic-rich stream, e.g., an ethylbenzene-rich stream.
The reference in the present specification to the ZSM-5 having a xe2x80x9ccrystal sizexe2x80x9d of no greater than 0.05 micron is used to mean that the maximum dimension of the crystals in any direction is no greater than 0.05 micron.